The recent growth of the Internet has created both a tremendous demand for additional subscriber access to public switched telephone networks (PSTN) and a demand for additional bandwidth for the access. The former demand is being met by deploying additional analog access lines so that modems can be used for the Internet access, and the latter demand is being met by deploying integrated service digital network (ISDN) lines. ISDN is the principal technology for placing a digital signal onto a copper pair that originally provides only analog dial tone, and it was developed in the 1980's, when state-of-the-art digital encoding technology resulted in the standards as described in Bellcore documents TR-TSY-000393 and TR-TSY-000397. The basic transmission rate, called BRI for ISDN is 160 kilobits per second (kbps). This digital rate and its corresponding communication method are called digital subscriber line (DSL).
In the case of modems, although the Internet access is provided, insufficient bandwidth limits the graphic service to still images which take seconds or minutes to download; while, in the latter case of ISDN, a subscriber's main standard plain old telephone service (POTS) line is not converted to ISDN in order to maintain the subscriber's POTS access in the event of a local power outage or electronic system failure because the ISDN service is dependent upon the residence power and proper function of the ISDN electronics; therefore, additional copper cable pairs need to be installed. In addition, the ISDN transmission with BRI only modestly increases the download speed of still images in comparison to modems. They can not provide the full graphic-rich service besides "jerky videos;" therefore, a transmission speed higher than BRI is desired.
In both cases, the installation of an additional copper-pair based access line is not financially viable for the telephone companies due to the long depreciation schedule for these cables. It is generally recognized that a higher-bandwidth media, such as fiber optic cable, is the ultimate solution for the digital access though the fiber connectivity precludes the lifeline access in the event of local power failures. While the technical and financial issues related to fiber installation are being worked out, installing copper cables only consumes capital and delays the day for fiberization.
One approach to avoid installing the additional copper pair cables is to more efficiently utilize existing phone lines for high-speed digital transmissions. The phone lines are made of twisted copper pairs and are configured in a star-like architecture that is suitable for bi-directional communications. The existing copper cable outside plant was constructed in accordance with design rules specifying that for local loops exceeding 13 kilo-ohms (k.OMEGA.), or approximately 18 kilo-feet (kft) which is equivalent to 5,486 meters, loading coils or filter capacitors are added to remove voice frequencies shifted above 4 kilo-Hz (kHz) due to the loop resistance. The REA loop survey of 1986 indicates that for the US as a whole, approximately 15% of all loops are loaded. Since ISDN uses a digital signal operating at a center frequency of 40 kHz, it will not transmit in the presence of a load coil or capacitor. Bridged taps or branches attached to a primary cable run further reduce the reach of an ISDN signal over existing copper cables, with the net result being that approximately 70% of all existing subscribers can have ISDN service added without additional construction expenses, as reported by Pacific Bell in early 1996. But the question remains how the existing cables can be used to carry both a POTS signal and an ISDN signal with a transmission rate faster than BRI.
Since 1990, the performance of communication chipsets has significantly improved due to the development of microprocessors. High-bit-rate subscriber line (HDSL) chipsets can run at 784 kbps or 1 Mbps to transport one half of a T1/E1 digital loop carrier signal in an application called "Repeaterless T1/E1." Other types of high speed communication technologies for twisted pairs, such as asymmetric DSL (ADSL), are emerging from labs but are still too expensive for wide range applications.
HDSL technology, when reduced to a lower transmission speed than that in the T1 application, can be applied to transport one high speed digital signal and one POTS signal over the existing phone lines through multiplexing and demultiplexing, which can provide about 70% phone subscribers in the US for high speed Internet access while maintaining their old phone services without adding new twisted copper pairs or incurring any new expenses. This approach of multiplexing and demultiplexing was explored by several inventors in the past.
By way of example, Carse et al., U.S. Pat. No. 4,730,311 describe a multiplexer for use in a telephone system in which a plurality of subscriber locations are connected to a central office by a single subscriber loop. Carse et al. focus on the design of the multiplexer rather than the entire communication system. Their technique applies generally to any methods of digital transmission, consequently the transmission rate is arbitrary. The subscribers are defined to be locally powered and backed-up with battery power. The battery back-up can only last for a limited period of time in the case of local power loss. For the design of the multiplexer, Carse et al. do not define either a digital interface or standard of loopback testing. Also, the configuration of the central office is not described.
Litteral et al., U.S. Pat. No. 5,247,347 and Coddington et al., U.S. Pat. No. 5,410,343 define how to provide digital video signals from a video information provider to one or more of a plurality of subscriber premises. However, the multiplexers used in both systems mainly perform frequency domain multiplexing/demultiplexing which is inherently disadvantageous with respect to time domain multiplexing/demultiplexing. The power source of the multiplexers is not specified. In addition, Litteral et al. and Coddington et al. only describe transport and encoding of specific video signals rather than generic digital signals.
Bliven, U.S. Pat. No. 5,459,729 describes a method and apparatus for transmitting and receiving multiple telephone signals over a single twisted pair. Two conventional telephone signals are converted into one digital signal and then transported over a single twisted pair at a rate of 160 kbps. Creating a multiplicity of telephone channels in this way is sufficient for analog POTS but is too low to provide adequate Internet access.
Accordingly, it is a primary object of the present invention to provide a communication system that transports one high speed digital signal and a POTS channel over a single twisted cable pair, typified by the large non-loaded majority of the existing cable plant. It is a further object of the invention to provide line powering to a remote terminal to avoid dependence upon local power and to provide for a metallic POTS access in the event of electronic failures. This invention is subsequently referred to as a "HDSL and POTS carrier system" or abbreviated as "HPCS."